1. Field of the Invention
The present invention relates to an optical element which exhibits a change in incidence angle and a scanning optical system having the optical element. The present invention is suitable for an image forming apparatus such as a laser beam printer or digital copying apparatus which has, for example, an electrophotographic process of deflecting a light beam emitted from a light source means by using a light deflector (deflecting means), and recording image information by optically scanning a scanning target surface through a scanning optical means including an optical element having f-θ characteristics and a microstructure lattice formed thereon.
2. Related Art
In a conventional scanning optical system such as a laser beam printer (LBP), a light beam which is optically modulated in accordance with an image signal and emitted from a light source means is cyclically deflected by a light deflector formed from, e.g., a polygon mirror, and the light beam is focused into a spot on the surface of a photosensitive recording medium and optically scanned by an imaging optical system having f-θ characteristics.
FIG. 13 is a sectional view (main scanning cross-section) of a conventional scanning optical system in the main scanning direction.
Referring to FIG. 13, a light source means 91 is formed from, for example, a semiconductor laser or the like. A collimator lens 92 converts a divergent light beam emitted from the light source means 91 into an almost parallel light beam. An aperture stop 93 shapes a beam shape by limiting a light beam that passes through. A cylindrical lens 94 has a predetermined power in only the sub-scanning direction and forms a light beam which passes through the aperture stop 93 into an almost line image on a deflecting surface (reflecting surface) 95a of a light deflector 95 (to be described later) within a sub-scanning cross-section.
The light deflector 95 serving as a deflecting means is formed from, for example, a polygon mirror (rotating polyhedral mirror) having a tetrahedral arrangement. The light deflector 95 is rotated by a driving means (not shown) such as a motor at constant speed in the direction indicated by an arrow A in FIG. 13.
A scanning lens system 96 serves as a scanning optical means having a focusing function and f-θ characteristics and is formed from first and second scanning lenses 96a and 96b. The scanning lens system 96 forms the light beam based on image information, which is reflected/deflected by the light deflector 95, into an image on a photosensitive drum surface 97 serving as a scanning target surface, and has an optical face tangle error correction function of making the deflecting surface 95a of the light deflector 95 and the photosensitive drum surface 97 have a conjugate relationship within a sub-scanning cross-section.
Referring to FIG. 13, the divergent light beam emitted from the semiconductor laser 91 is converted into an almost parallel light beam by the collimator lens 92, and the light beam (light amount) is limited by the aperture stop 93. The resultant light beam is incident on the cylindrical lens 94. Of the substantially parallel light beam incident on the cylindrical lens 94, the light in a main scanning cross-section emerges without any change. The light in a sub-scanning cross-section is focused and substantially formed into a line image (elongated in the main scanning direction) on the deflecting surface 95a of the light deflector 95. The light beam reflected/deflected by the deflecting surface 95a of the light deflector 95 is formed into a spot on the photosensitive drum surface 97 via the first and second scanning lenses 96a and 96b. This light beam is then scanned on the photosensitive drum surface 97 at constant speed in the direction indicated by an arrow B (main scanning direction) by rotating the light deflector 95 in the direction indicated by the arrow A. With this operation, an image is recorded on the photosensitive drum surface 97 as a recording medium.
The above conventional scanning optical system, however, has the following problems.
Recently, the scanning optical means (scanning lens system) of a scanning optical system is generally made of a plastic material that allows easy formation of an aspherical shape and is easy to manufacture. It is, however, difficult in terms of technique and cost to form an antireflection coat on the lens surface of a plastic lens. As a consequence, Fresnel reflection occurs on each optical surface.
FIG. 14 is a graph for explaining the angle dependency of reflectance and transmission when a P-polarized light beam is incident on, for example, a resin optical member having refractive index n=1.524. As shown in FIG. 14, the surface reflection on each optical surface increases with an increase in incidence angle.
The first problem is therefore that surface reflected light on a lens surface without an antireflection coat is reflected by other optical surfaces and finally reaches a scanning target surface to produce ghosts. If one of the two scanning lenses which is closer to the light deflector has a concave lens surface and a light beam incident thereon is nearly vertical, Fresnel reflected light on this lens surface returns to the light deflector and reflected by the deflecting surface (reflecting surface) of the light deflector. This reflected light passes through the scanning optical means and reaches the scanning target surface to become a ghost.
The second problem is that since the incidence angle of a light beam incident on the scanning optical means generally changes as it travels from an on-axis position (scanning center) to an off-axis position (scanning periphery), Fresnel reflection on each optical surface greatly changes to produce a difference between the light amount at an on-axis position and that at an off-axis position.
FIG. 15 is a graph showing a transmission on each surface when P-polarized light beam is incident on the scanning optical means in FIG. 13. As shown in FIG. 14, since the reflectance decreases (transmission increases) with an increase in incidence angle, the transmission of the overall system increases from an on-axis position to an off-axis position. That is, the illuminance distribution on the scanning target surface also increases from an on-axis position to an off-axis position.
According to the graph of FIG. 15, the light amount at the outermost off-axis position is larger than that at an on-axis position by 5%. As a result, the image output from the image forming apparatus has a density difference between a central portion and a peripheral portion.
An attempt has been made to solve this problem by adjusting the diffraction efficiency of a diffraction grating placed in a scanning optical means as in Japanese Patent Application Laid-Open No. 2000-206445. More specifically, a lattice is formed at a desired pitch with a desired power distribution to realize magnification chromatic aberration correction or focus correction, and the lattice height (depth) of the diffraction grating surface is properly set to change the diffraction efficiency of diffracted light (1st-order diffracted light) to be used at an on-axis position and an off-axis position, thereby canceling out a change in transmission on other refracting surfaces.
In this method, however, as the diffraction efficiency of diffracted light to be used is reduced, diffracted light of another order (to be also referred to as unnecessary diffracted light) increases. The increased diffracted light of another order reaches the scanning target surface to become flare light to cause image deterioration unless the light is shielded by using a light-shielding wall or the like.